Electro deposited magnetic films

ABSTRACT

A process for improving the quality of a main magnetic film formed on a thick conductive substrate wherein a thin, fine grain, magnetic smoothing film of nickel-phosphorus is first deposited on the substrate and then the main magnetic film is formed on the smoothing film. The product formed comprises at least one main magnetic film formed on a thin fine grain, magnetic nickel-phosphorus smoothing layer. The main magnetic film is at least three times as thick as the smoothing layer. The smoothing layer contains from 0.5 to 8 percent phosphorus with the balance nickel.

llnited States Patent Redwood City, Calif.

[54] ELECTRO DEPOSITED MAGNETIC FILMS 1 Claim, No Drawings [52] U.S. Cl 204/40,

29/183.5, 29/194, 29/196.6, 204/43, 340/174 [51] Int. Cl C23b 5/50 [50] Field of Search 340/ 174 QA; 204/41, 40, 43; 29/194, 183.5, 196.6; 117/239, 71 R, 71 M [56] References Cited UNlTED STATES PATENTS 3,327,297 6/1967 Croll 117/71 M X 3,355,267 11/1967 DuRose.... 29/194 3,393,982 7/1968 Fisher et a1.. 29/194 3,524,173 8/1970 Wolf 204/41 X Primary ExaminerG. L. Kaplan Attorney-Robert G. Clay ABSTRACT: A process for improving the quality of a main magnetic film formed on a thick conductive substrate wherein a thin, fine grain, magnetic smoothing film of nickelphosphorus is first deposited on the substrate and then the main magnetic film is formed on the smoothing film. The product formed comprises at least one main magnetic film formed on a thin fine grain, magnetic nickel-phosphorus smoothing layer. The main magnetic film is at least three times as thick as the smoothing layer. The smoothing layer contains from 0.5 to 8 percent phosphorus with the balance nickel.

ELECTRO DEPOSITED MAGNETIC FILMS BACKGROUND OF THE INVENTION Thin magnetic films are employed in many memory devices, such as closed flux structures, open flux structures, plated wires and the like. When a thin magnetic film is electrodeposited onto thick or large grained conducting substrates they have been found to be of poor quality, in that they had very large easy access dispersion when the films are of the order of 1,000 A or less in thickness. The poor quality of the prior art thin films has been attributed to the fact that a good deal of interaction takes place between the crystals in the metallic substrate and the atoms deposited onto the substrate. Thus, crystalline anisotropy is reported to play some role in increasing dispersion. In addition, it is well known that microscopic roughness of substrates generally leads to dispersion. Since it is very difficult to achieve a microscopically smooth surface on a metallurgically prepared metal, the deposition of good magnetic thin films on such surfaces has not been possible.

Accordingly, to provide suitable structures in prior art devices, wherein the magnetic films deposited are of sufficiently high quality for use in fast, large magnetic memories, various intermediate layer materials have been utilized. However, the results are generally unsatisfactory due to the difficulty of photoetching the sandwich configuration in a single step process, which would be a highly desirable improvement. The properties required of the intermediate layer material are that it must be conductive photoetchable with an etchant which would not severely undercut the magnetic layer, and sufficiently fine grained such that a low-dispersion, high-quality magnetic film may be superimposed upon the material.

In the past, it has been proposed to remedy this by the use of nonmagnetic layer of nickel-phosphorous which acts as a microscopic smoothing" layer of the material used as a substrate. However, it has been found that in order to make the layer nonmagnetic, it is necessary that the layer contain at least 8 percent phosphorus. The relatively high-phosphorus content makes the material difficult to etch with the usual etchants used for materials such as copper or permalloy.

SUMMARY OF THE INVENTION The present invention provides a process, and a group of materials, for depositing a magnetic material to define a fine grain main magnetic layer of the order of, for example, 400 to 4,000A thick. By way of definition, fine grain is intended herein as a grain size equal to or less than a domain wall width. An intermediate magnetic layer effects a microscopic smoothing" of any material used as a substrate, such that when the main magnetic film is deposited onto the smoothing layer it is sufficiently isolated from the effects of the substrate material to greatly improve the magnetic characteristics of the deposited main magnetic film.

It was previously thought that in order for the smoothing layer to function properly, it was necessary that this layer be nonmagnetic. It has now been found that it does not interfere with the operation of the main magnetic layer if the smoothing layer itself is magnetic providing that the smoothing layer is not over one-third the thickness of the main magnetic layer. This is achieved by lowering the quantity of phosphorus which renders the film magnetic but much easier to etch. The resulting main magnetic film has the well-defined anisotropy and low-dispersion characteristics of conventional high-quality magnetic films, which films are presently obtained only by depositing them on smooth surfaces such as glass. The capability of producing high-quality magnetic films directly on conductor substrate materials leads to an enormous relaxation of the complexity of fabricating thin magnetic film structures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In carrying out the present invention, it is first necessary to have a suitable substrate. This can be any of the usual sub strates such as glass, polyester film or metal. If the substrate material is not conductive, such as in the case of glass or polyester films. a conductive layer is first deposited thereon by a conventional sputtering or evaporating process. Naturally if the substrate is metal, no such conductive layer is necessary and polishing is sufficient preparation. After the substrate is prepared, it is placed in a first magnetic plating solution and the thin, smoothing layer of the present invention deposited thereon as is hereinafter described in detail. After the magnetic smoothing layer is deposited, the main magnetic layer is then electrodeposited thereon utilizing conventional techniques. Naturally, the process of the present invention can be applied to more than one layer so that in building up a sandwich structure, alternate magnetic smoothing layers,

5 main magnetic layers and conductive layers can be applied one over the other. The present invention particularly lends itself to the production of sandwich structures which are subsequently etched to provide a number of memory structures on a single substrate.

In those structures which are etched, the present invention is particularly advantageous since the etching conditions can be milder and little or no nickel phosphide remains after etching. However, the present invention is not limited to those structures which are eventually etched and the smoothing layer of the present invention can be employed in any situation wherein one wishes to provide a high-quality magnetic layer.

The preferred process of the invention utilizes a nickelphosphorus material as the magnetic fine grain film 16, wherein the composition contains only from one-half to 8 percent phosphorus with the balance nickel. Accordingly, the associated bath for the nickel-phosphorus plating procedure is formed of a solution containing nickel ions and phosphorus compounds such as the hypophosphite ion.

The nickel-phosphorus film may be deposited, for example, from any of the plating baths well known to those skilled in the art such as sulphate and sulphamate baths. By way of example, the following table I gives the composition of a nickel-sulphate chloride bath by which the nickel-phosphorus film may be deposited.

TABLE I NiSO -6H,O 200 g./l. NaCl l0 g./l. NaBO l5 g./l. NaH PO 'H O 3 g./l. Saccharin 8 g./l. Sodium Lauryl Sulfate l g./l.

Plating is carried out at current densities 25-200 ma./cm.

Table II illustrates a sulfamate bath which can be used in the same manner as the bath of table I.

TABLE II Ni(added as sulfamate) 50 g.ll. NaCl 15 g./l. NaBO; l5 gJl. Saccharin 5 g./l. Sodium Lauryl Sulfate l g./l. NaH,-P0 -H 0 3 g./l.

As has been stated previously, the composition of the resulting film must have at least one-half percent phosphorus and may contain up to 8 percent phosphorus. It has been found that if the film has less than one-half percent phosphorus, the desired smoothing effect is not obtained and the resulting main magnetic film thus does not have the desired properties. On the other hand, if one goes above 8 percent of phosphorus, a high-quality ultimate film can be produced from a magnetic standpoint, but the resulting film is difficult to etch and it is therefore imperative that the composition not contain over 8 percent phosphorus in order to achieve the desired results.

The thickness of the magnetic smoothing layer is also critical. It has been found that if the layer is thinner than 50A the desired smoothing is not obtained. At about 200A the effects of the film almost level off, although the film can be as thick as 1,000A with slightly improved results. In any event, in order that the smoothing layer in itself not act as a magnetic layer and thus detract from the properties of the ultimate magnetic not intended to limit the invention except as defined in the following claims.

We claim:

1. A process for depositing a main anisotropic magnetic film layer, it is necessary that the smoothing layer be not thicker 5 onto a thick large grain conductive substrate comprising the than one-third of the thickness of the ultimate magnetic layer. Since many storage elements include very thin magnetic layers such as 400A, it is obviously preferred to employ the thinnest possible smoothing layer in order to provide the desired ultimate structure wherein the smoothing layer does not exert an adverse effect on the ultimate magnetic film. Thus smoothing layers offrom about 50 to 200A are preferred.

Although the invention process and product has been described herein with reference to several particular embodiments, it is to be understood that various modifications may be made thereto within the spirit of the invention, and thus it is steps of:

a. electroplating a smoothing film of fine grain magnetic material having a grain size equal to or less than a domain wall width consisting of from 99.5 to 92 percent nickel and from one-half to 8 percent phosphorus on the substrate having a thickness offrom 50 to 200A;

b. electroplating said main anisotropic magnetic film on said smoothing film, and

c. said main film being at least three times as thick as the smoothing film. 

